1. Field of the Invention
The invention lies in the field of electronic circuits using insulated-gate field-effect transistors to obtain current sources. These circuits use so-called MOS technology and are generally in the form of integrated circuits or are part of integrated circuits. The invention relates more specifically to current sources of this type that are designed to display a certain degree of immunity to temperature variations.
2. Description of the Prior Art
Current sources generally have many applications in electronics. They are used notably to make calibrated ramp signal generators. For this purpose, the current source supplies a capacitor whose voltage gives the ramp signal.
Ramp generators are used, for example, to carry out the programming or erasure of memory cells constituting electrically erasable programmable memories (EEPROMs).
A known assembly in MOS technology for making a current source consists of the use of two current mirrors respectively using p channel MOS (PMOS) transistors and n channel MOS (NMOS) transistors, the NMOS transistors having different threshold values (see the diagram of FIG. 1). It can be shown that the currents flowing in the arms of this circuit are approximately proportional to the carrier mobility of the NMOS transistors and to the square of the difference of their threshold values. The result thereof is that the currents are in fact highly dependent on the temperature because the carrier mobility as well as the square of the difference between the threshold values varies very greatly as a function of the temperature.
The problem of the temperature stabilization of electronic circuits in general is known per se but usually leads to making the circuits more complicated and to increasing their consumption.
Hence, an aim of the invention is to propose a simple and efficient approach to this problem in the case of current sources.